Weighing cells of the abovementioned type have the disadvantageous characteristic that they produce measured-value error due to vibration of their place of installation. The cause for this are the forces, which due to the mass acceleration act on the mechanical elements of the scales and on the mass to be weighed. Weighing cells of the type in question here, namely those which operate practically without displacement, are substantially more exposed to such interferences than displacement (odometrical) systems, as for example spring scales.
The abovedescribed weighing cells also must operate satisfactorily at places at which they are exposed to strong vibrations. If one wanted to eliminate the effect of vibrations only by accordingly structuring the scale, namely by dynamically balancing all lever elements of load introduction and measuring transformation, by referring their centers of gravity to one axis or by bringing same to congruency, this would mean a very considerable amount of expense, which in addition would not result in a 100% success. The other possibility consists in eliminating the interferences in the measuring signal by using suitable filter circuits. The utilization of such filters, however, has the disadvantage of undesirably long measuring times.
Weighing cells of the type in question here are also utilized in packaging machines and automatic control carriers. This means that the measuring times must be very short. These times are supposed to be less than 0.1 s. A weighing cell with electromagnetic force compensation, which operates practically without displacement and in which all effective elements, including the weighing dish and the measuring device, have mass, represents an almost ideal acceleration receiver for vertical, translatory vibrations. This is particularly true for low interfering frequencies. On the other hand, especially low interfering frequencies, when electronic filters are connected at the output end, require a long centering (balancing) time for the measuring operation.
An electrical weighing device for moving loads is known from German Patent No. 17 74 137, in which a load-measuring cell has an acceleration meter associated with it such that it experiences the same vertical accelerations. Load cell and acceleration meter are disposed in the same electrical circuit and are connected together such that the error (spurious) factor caused by the acceleration is supposed to be eliminated. Such a scale is supposed to be used on ships, in order to be able to weigh the catch even while in sloping positions due to high sea motion. The particularity of this solution consists in the acceleration meter changing the voltage which is delivered to the load cell, whereby the acceleration meter is supposed to be connected in series with the output circuit of the load cell. In the exemplary embodiment, the hanging scale is designed according to the wire strain gauge principle, namely same forms a quasi-force meter, while the acceleration meter is conceived as a potentiometric displacement pickup. It is here tried to compensate the output signal of a scale, which shows an acceleration meter for vibrations, with the output signal of an odometer. This functions satisfactorily only at a very special frequency. Moreover, the acceleration compensation acts only for a particular load drop.
The basic purpose of the invention is to construct a weighing cell of the abovementioned type such that on the one hand even at low interfering frequencies a satisfactory measuring result is obtained within the shortest time and that on the other hand the compensation is maintained automatically for the entire measuring range of the weighing cell.
This is attained according to a first solution of the purpose by the weighing signal produced by the load cell, and which signal is superposed by the spurious (error) signal, being fed to a subtractor, by the signal produced by the acceleration meter being balanced to a value and being fed to a multiplier, by the cleaned signal which comes from the subtractor being fed to the multiplier, and by the signal which is obtained from the multiplier being fed to the subtractor.
A second solution of the purpose of the invention is characterized by the weighing signal superposed by the spurious signal being fed to a subtractor, to which subtractor is also fed the signal from the acceleration meter after the latter signal is amplified and balanced such that the additive weight-force component is balanced, by a second signal which is produced by the acceleration meter and is balanced such that the multiplicative weight-force component is compensated and thereafter is fed to a second subtractor, by a reference voltage also being fed to the second subtractor, and by the outputs of both subtractors being fed to an analogue-digital converter which delivers the cleaned output signal.
The spurious (error) signal (a), which starts out from the load cell, and which signal is superposed by the weighing signal (A), is caused on the one hand by a constant mass portion of the weighing cell, namely by the levers, the weighing dish, etc. and on the other hand has a part which depends on the load. The signal (a) which is delivered for the compensating of the acceleration meter must also necessarily consist of an additive and a multiplicative component. The adjustment of the compensation signal (a) which is delivered by the acceleration meter must thus also be adjusted to the respective mass which is placed onto the weighing cell. This can be achieved according to the invention by both the signal produced by the subtractor and also the signal produced by the acceleration meter and which is if desired amplified being fed to a multiplier, whereby the product of these two signals must equal the spurious signal (a). Said signal (a) is then in turn fed to the subtractor. This circuit of subtractor and multiplier acts in the form of a dividing building block. This solution has proven to be especially advantageous, since on the one hand a balancing for any desired load exists and on the other hand the cleaned signal (A) is used as a multiplier factor.
It is possible to achieve with the inventively constructed measuring cell an important improvement in the interference behaviour by a factor of 20 or more.
Further advantageous developments of the invention can be taken from the subclaims in connection with the description and drawings.